1. Field of the Invention
The present invention relates to a steam turbine, which is capable of cooling or heating a turbine rotor by air or the like, and a method of cooling a steam turbine.
2. Description of the Related Art
Generally, during a start-up of a steam turbine, a turbine rotor, lots of parts of which are directly exposed to high-temperature steam, has a quick temperature increase, while a casing, having a large thermal capacity, has a slow temperature increase. Here the casing means stationary portions of the steam turbine.
FIG. 20 is a diagram showing a thermal expansion difference, which is a difference between an axial expansion amount of the turbine rotor and an axial moving distance of seal fins of a labyrinth portion due to an axial expansion of the casing during the start-up of the steam turbine.
As shown in FIG. 20, during the start-up of the steam turbine, the turbine rotor rotates at a low speed even if the mainstream of steam has a small flow rate, its temperature is increased gradually by windage loss or the like, and an axial expansion amount of the turbine rotor is increased. Meanwhile, since a casing part including the labyrinth portion has a large thermal capacity, the temperature increase becomes moderate. Therefore, a temperature difference is produced in the casing part including the turbine rotor and the labyrinth portion, a difference (thermal expansion difference) is generated between an axial expansion amount of the turbine rotor and an axial moving distance of the labyrinth portion due to an axial expansion of the casing. This thermal expansion difference increases with the lapse of time from the start-up to indicate a maximum value (maximum thermal expansion difference) and decreases as it approaches the rated condition.
Thus, when the thermal expansion difference is produced in the axial direction of the turbine rotor by the turbine rotor and the seal fins of the labyrinth portion, there is a possibility that protruded threads formed on the circumferential surface of the turbine rotor and the seal fins of the labyrinth portion are contacted (rubbing).
To avoid such a contact, the conventional steam turbine increases the gap of the labyrinth portion or increases the temperature over a long time so that the temperature difference between the turbine rotor and the casing part including the labyrinth portion becomes small at the start of the steam turbine. But, to improve the performance of the steam turbine in recent years, there is a desire for a decrease in the gap of the labyrinth portion or for a decrease in the start-up time of the steam turbine to decrease the waiting time of a quick start-up gas turbine of a combined cycle plant.
To decrease the thermal expansion difference between the turbine rotor and the casing part including the labyrinth portion, it is necessary to decrease the individual temperature differences. To do so, it is considered to heat the casing which is slow in temperature increase or to cool the turbine rotor which is quick in temperature increase. When the steam turbine is stopped, a decrease in temperature of the casing having a large thermal capacity becomes moderate, and a decrease in temperature of the turbine rotor having a small thermal capacity becomes quick.
As a steam turbine which prevents a contact of the labyrinth portion due to such a thermal expansion difference, for example, JP-A 2006-17016 (KOKAI) discloses a technology of heating the casing by steam with a steam passage disposed in a flange portion, whose temperature increase is most pronounced at the start of the steam turbine in the casing.
But, since the above-described conventional steam turbine, which heats the flange portion of the casing by steam at the start of it, is provided with a mechanism of heating the casing, it has a disadvantage that the casing has a complex structure. Since a large amount of steam was required to heat the casing having a large thermal capacity, improvement of the steam turbine efficiency was hindered. Besides, time for heating the casing having a large thermal capacity is required, and there was a problem that it took time to start the steam turbine.